drm/i915/gt: Fixup config ifdeffery for pm_suspend_target_state

drm/i915/gt: Fixup config ifdeffery for pm_suspend_target_state

pm_suspend_target_state is declared under CONFIG_PM_SLEEP but only
defined under CONFIG_SUSPEND. Play safe and only use the symbol if it is
both declared and defined.

Reported-by: kbuild-all@lists.01.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: a70a9e998e8e ("drm/i915: Defer rc6 shutdown to suspend_late")
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120182209.3967833-1-chris@chris-wilson.co.uk